Lawyers for the government have admitted that a senior MI5 officer gave false evidence to the high court in the case of former Guantánamo Bay prisoner Binyam Mohamed.

The admission, combined with an apology, is contained in a letter from Treasury solicitor David Mackie to Lord Justice Thomas and Mr Justice Lloyd Jones, who tomorrow will hear fresh demands from lawyers representing Mohamed, and the media, for the disclosure of information about who was complicit in his interrogation and torture.

The letter reveals that an MI5 officer, referred to as Witness A, gave "incorrect" evidence to the high court about when the CIA kept British security and intelligence officers informed about Mohamed's secret interrogation.

The officer told the court MI5 had no further contact with the CIA about Mohamed after February 2003, when he was being held in Morocco. It is still unclear how much MI5 knew about Mohamed's continuing interrogation in Morocco, where he was held before being transferred in May 2004, 15 months later, to the CIA's "dark prison" in Bagram, Afghanistan.

Mohamed was flown to Guantánamo Bay in September 2004 and released in February this year. MI5 says it did not know of his whereabouts when he was being mistreated.

Mackie also apologised to the high court for not handing over 13 documents which he says should have been passed to the judges. They included six MI5 documents and MI6 documents identified originally by the security and intelligence agencies but subsequently not "selected for disclosure".

They were rediscovered after the intelligence and security committee, made up of MPs and peers, agreed in February to set up its own investigation into the case after prompting by the high court.

Lawyers for Mohamed and the media, led by the Guardian, will urge the high court tomorrow to release at least a summary of 42 documents which shed light on what MI5 knew about the treatment of Mohamed and other detainees. It is believed they will show who in government, including officials in 10 Downing Street, implicitly approved of the treatment.

Most of the documents originated in the US. David Miliband, the foreign secretary, has argued that their release would damage US-UK relations. He claimed the US would stop sharing intelligence with Britain if a UK court ordered their disclosure.

Police from Scotland Yard's specialist crimes directorate are investigating "possible criminal wrongdoing" by another MI5 officer, known as Witness B, who interrogated Mohamed in a Pakistani jail shortly after he was seized in 2002.

Former shadow home secretary David Davis yesterday described the government lawyers' letter to the high court as "extraordinary". He added: "The state is using secrecy to cover up embarrassment."